1. Field of the Invention
This invention relates to selected multifunctional terminally unsaturated urethane oligomers useful for curing by radiation such as UV light or electron beam. The present invention also relates to the process for making these oligomers as well as radiation curable polymer formulations containing these oligomers.
2. Description of the Prior Art
Environmental advantages, production efficiencies, and low-cost plant installations continue to spur growth for radiation curable materials. With this growth, this is an awareness that better radiation curable materials are needed for specific applications.
Terminally unsaturated urethane oligomers, more commonly called capped urethane oligomers, have been used in such radiation curing applications. These capped urethanes are readily available and have found widespread commercial use. Like conventional urethanes, they possess toughness, wear resistance, adhesion, and flexibility besides being curable by UV light.
Several prior art methods for making capped urethanes have been disclosed:
(1) reaction of a polyol with a polyisocyanate followed by capping that intermediate product with a hydroxyl-functional acrylate or hydroxyl-functional vinyl ether; or
(2) reaction of a hydroxyl-functional acrylate or hydroxy-functional vinyl ether with a polyisocyanate followed by reaction of that intermediate product with a polyol; or
(3) simultaneous reaction of a polyol, a polyisocyanate, and a hydroxyl-functional acrylate or hydroxyl-functional vinyl ether.
These capped urethane oligomer products are then rapidly polymerized by the free radicals or cations generated by exposure to radiation such as ultraviolet light or electron beams.
The overall chemical formula of these terminally unsaturated urethane oligomers is the single most important factor for their performance in radiation-curing applications because it is primarily responsible for the basic polymer properties required in the overall product. While reactive monomers can be blended with these oligomers for viscosity reduction, both the type and amount of monomer have less affect on the desired properties of the final cured product than these oligomers. More specifically, the oligomer viscosity and its ease of reduction with minimal quantities of reactive monomers is of critical importance in achieving certain processing applicability. An excellent discussion of how viscosity and other characteristics are effected by the design of the oligomer is presented by B. Martin, Radiation Curing, 13(3), 4 (1986).
Since viscosity of the oligomers is, in general, a function of increasing molecular weight, rigid films formed from the radiation curing of low molecular weight capped urethane oligomers with low solution viscosity can approach the properties of conventional rigid urethanes. However, flexible films obtained in radiation curing have not performed as well as conventional moisture cured or two-component urethane counterparts.
There is, therefore, a constant challenge in the radcure industry to achieve oligomer performance more similar to that obtainable by conventional flexible urethanes while maintaining a low enough viscosity to permit practical use. Hodakowski et al., U.S. Pat. No. 4,131,602, describes an approach to lowering viscosity by blending various polyols in making the acrylated urethane oligomers. A second patent by Watson, Jr., U.S. Pat. No. 4,246,391, describes a process modification wherein the diisocyanate is first reacted with a hydroxyl acrylate before adding a polyol. However, the reaction products in each patent were both highly diluted with a reactive monomer.
Another approach to improve oligomer performance is to increase the functionality of the oligomer beyond two functional by reacting a multi-functional modifying group (e.g., a polyhydric alcohol) with the acrylated urethane oligomer. In general, this approach improves the tensile strength of the final polymer and gives improved solvent resistance. Again, while properties can be improved, the increased functionality also increases viscosity which limits the useable concentration of oligomer in a final formulation. A discussion on the effects of oligomer functionality is presented in J. McConnell et al., ACS Symposium, Radiation Curing of Polymeric Materials (1989).
The present invention is different from the above-noted traditional technologies because it provides an approach whereby oligomer functionality and urethane character are both increased while viscosity is decreased.